This invention relates generally to an exposure control for printing photographic copies and particularly relates to such a control which permits to automatically adjust the exposure time in accordance with variations of certain parameters such as the lens aperture of the printer or the magnification ratio.
Many devices are presently known for determining the illumination level or light intensity which exists at the surface of a photographic material to be printed. By means of these devices the operator is able to adjust the light intensity, as well as the exposure time to provide a print which has been properly exposed without time consuming and expensive trial and error methods. Also, automatic exposure timers are generally known and widely used.
By means of a conventional light meter and timer it is possible to prepare excellent prints without trial and error. However, in general, the operator must have available two items of information. In the first place, he must know the light intensity at the plane of the projected image. Furthermore, he must know the sensitivity of the photographic material he has selected.
With this information available the operator must now correlate the sensitivity or "speed" of the photographic material with the readings of the light meter. This will then enable him to read the exposure time from the timer setting of the printer. Then the light meter is removed from the area of the image, replaced by the photographic material and the timed exposure cycle is started.
Some instruments do not read directly in terms of exposure time, but provide a multiplier which is to be used with the known exposure time. Therefore, the operator must calculate the new exposure time and enter it upon the setting of the timer of the printer.
If the operator selects a higher than normal magnification ratio, or a smaller lens aperture than the standard or normal one which may, for example, be done for purposes of distortion control, then the new exposure multiplier must be tediously recomputed before the print is made.
It is accordingly an object of the present invention to provide an exposure control apparatus which combines the functions of a conventional light meter with that of a conventional timer control, thereby to obviate the necessity of calculating exposure times.
A further object of the present invention is to provide an exposure control which is automatic and which operates both with black and white and color prints and which includes electronic controls.
Yet another object of the present invention is to provide an exposure control of the type where the operator must only enter a standard time corresponding to the speed of the selected photographic paper and can then adjust for variations of the lens opening, the magnification ratio and other parameters to automatically obtain the necessary timing signal.